1. Field of the Invention
The present invention relates to improved thermoelectrics for producing heat and/or cold conditions with a greater efficiency.
2. Description of the Related Art
Thermoelectric devices (TEs) utilize the properties of certain materials to develop a thermal gradient across the material in the presence of current flow. Conventional thermoelectric devices utilize P-type and N-type semiconductors as the thermoelectric material within the device. These are physically and electrically configured in such a manner that the desired function of heating or cooling.
Some fundamental equations, theories, studies, test methods and data related to TEs for cooling and heating are described in H. J. Goldsmid, Electronic Refrigeration, Pion Ltd., 207 Brondesbury Park, London, NW2 5JN, England (1986). The most common configuration used in thermoelectric devices today is illustrated in FIG. 1. Generally, P-type and N-type thermoelectric elements 102 are arrayed in a rectangular assembly 100 between two substrates 104. A current, I, passes through both element types. The elements are connected in series via copper shunts 106 soldered to the ends of the elements 102. A DC voltage 108, when applied, creates a temperature gradient across the TE elements. FIG. 2 for flow and FIG. 3 for an object both illustrate general diagrams of systems using the TE assembly 100 of FIG. 1.
When electrical current passes through the thermoelectric elements, one end of the thermoelectric elements becomes cooler and the other end becomes warmer. TE""s are commonly used to cool liquids, gases and objects.
The basic equations for TE devices in the most common form are as follows:                               q          c                =                              α            ⁢                          xe2x80x83                        ⁢            I            ⁢                          xe2x80x83                        ⁢                          T              c                                -                                    1              2                        ⁢                          I              2                        ⁢            R                    -                      K            ⁢                          xe2x80x83                        ⁢            Δ            ⁢                          xe2x80x83                        ⁢            T                                              (        1        )            xe2x80x83qin=xcex1Ixcex94T+I2Rxe2x80x83xe2x80x83(2)                               q          h                =                              α            ⁢                          xe2x80x83                        ⁢            I            ⁢                          xe2x80x83                        ⁢                          T              h                                +                                    1              2                        ⁢                          I              2                        ⁢            R                    -                      K            ⁢                          xe2x80x83                        ⁢            Δ            ⁢                          xe2x80x83                        ⁢            T                                              (        3        )            
where qc is the cooling rate (heat content removal rate from the cold side), qin is the power input to the system, and qh is the heat output of the system, wherein:
xcex1=Seebeck Coefficient
I=Current Flow
Tc=Cold side absolute temperature
Th=Hot side absolute temperature
R=Electrical resistance
K=Thermal conductance
Herein xcex1, R and K are assumed constant, or suitably averaged values over the appropriate temperature ranges.
Under steady state conditions the energy in and out balances:
qc+qin=qhxe2x80x83xe2x80x83(4) 
Further, to analyze performance in the terms used within the refrigeration and heating industries, the following definitions are needed:                     β        =                                            q              c                                      q                              i                ⁢                                  xe2x80x83                                ⁢                n                                              =                      Cooling            ⁢                          xe2x80x83                        ⁢            Coefficient            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            Performance            ⁢                          xe2x80x83                        ⁢                          (              COP              )                                                          (        5        )                                γ        =                                            q              h                                      q                              i                ⁢                                  xe2x80x83                                ⁢                n                                              =                      Heating            ⁢                          xe2x80x83                        ⁢            COP                                              (        6        )            
From (4);                                                         q              c                                      q                              i                ⁢                                  xe2x80x83                                ⁢                n                                              +                                    q                              i                ⁢                                  xe2x80x83                                ⁢                n                                                    q                              i                ⁢                                  xe2x80x83                                ⁢                n                                                    =                              q            h                                q                          i              ⁢                              xe2x80x83                            ⁢              n                                                          (        7        )            xe2x80x83xcex2+1=xcex3xe2x80x83xe2x80x83(8)
So xcex2 and xcex3 are closely connected, and xcex3 is always greater than xcex2 by unity.
If these equations are manipulated, conditions can be found under which either xcex2 or xcex3 are maximum or qc or qh are maximum.
If xcex2 maximum is designated by, xcex2m, and the COP for qc maximum by, xcex2cm, the result is as follows:                               β          m                =                                            T              c                                      Δ              ⁢                              xe2x80x83                            ⁢                              T                c                                              ⁢                      (                                                                                1                    +                                          Z                      ⁢                                              xe2x80x83                                            ⁢                                              T                        m                                                                                            -                                                      T                    h                                                        T                    c                                                                                                                    1                    +                                          Z                      ⁢                                              xe2x80x83                                            ⁢                                              T                        m                                                                                            +                1                                      )                                              (        9        )                                                      β                          c              ⁢                              xe2x80x83                            ⁢              m                                =                      (                                                                                1                    2                                    ⁢                  Z                  ⁢                                      xe2x80x83                                    ⁢                                      T                    c                                                  -                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  T                                                            Z                ⁢                                  xe2x80x83                                ⁢                                  T                  c                                ⁢                                  T                  h                                                      )                          ⁢                  
                ⁢                  where          ;                                    (        10        )                                Z        =                                            α              2                                      R              ⁢                              xe2x80x83                            ⁢              K                                =                                                                      α                  2                                ⁢                ρ                            λ                        =                          Figure              ⁢                              xe2x80x83                            ⁢              of              ⁢                              xe2x80x83                            ⁢              Merit                                                          (        11        )                                          T          m                =                                            T              c                        +                          T              h                                2                                    (        12        )            
and;
Wherein:
xcex=Material Thermal Conductivity; and
xcfx81=Material Electrical Resistivity
Note that for simple solid shapes with parallel sides, K=xcexxc3x97area/length. Similarly R=(xcfx81xc3x97length)/area. Thus, any change in shape, such as a change in length, area, conality, etc., can affect both K and R. Also, if the shapes of flexible elements are changed by mechanical or other means, both K and R can change.
xcex2m and qcm depend only on Z, Tc and Th. Thus, Z is named the figure of merit and is basic parameter that characterizes the performance of TE systems. The magnitude of Z governs thermoelectric performance in the geometry of FIG. 1, and in most all other geometries and usages of thermoelectrics today.
For today""s materials, thermoelectric devices have certain aerospace and some commercial uses. However, usages are limited, because system efficiencies are too low to compete with those of most refrigeration systems employing freon-like fluids (such as those used in refrigerators, car HVAC systems, building HVAC systems, home air conditioners and the like).
The limitation becomes apparent when the maximum thermoelectric efficiency from Equation 9 is compared with Cm, the Carnot cycle efficiency (the theoretical maximum system efficiency for any cooling system);                                           β            m                                C            m                          =                                                                              T                  c                                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  T                                            ⁢                              (                                                                                                    1                        +                                                  Z                          ⁢                                                      xe2x80x83                                                    ⁢                                                      T                            m                                                                                                                -                                                                  T                        h                                                                    T                        c                                                                                                                                                1                        +                                                  Z                          ⁢                                                      xe2x80x83                                                    ⁢                                                      T                            m                                                                                                                +                    1                                                  )                                                                    T                c                                            Δ                ⁢                                  xe2x80x83                                ⁢                T                                              =                      (                                                                                1                    +                                          Z                      ⁢                                              xe2x80x83                                            ⁢                                              T                        m                                                                                            -                                                      T                    h                                                        T                    c                                                                                                                    1                    +                                          Z                      ⁢                                              xe2x80x83                                            ⁢                                              T                        m                                                                                            +                1                                      )                                              (        13        )            
Note, as a check if Zxe2x86x92∞,xcex2xe2x86x92Cm. The best commercial TE materials have Z such that the product;
ZTa≈1
Several commercial materials have a ZTa=1 over some narrow temperature range, but ZTa does not exceed unity in present commercial materials. This is illustrated in FIG. 4. Some experimental materials exhibit ZTa=2 to 4, but these are not in production. Generally, as better materials may become commercially available, they do not obviate the benefits of the present inventions.
Several configurations for thermoelectric devices are in current use for automobile seat cooling systems, for portable coolers and refrigerators, for high efficiency liquid systems for scientific applications, for the cooling of electronics and fiber optic systems and for cooling of infrared sensing system.
All of these devices have in common that the Th is equalized over the hot side of the TE, and similarly, Tc is equalized over the cold side. In most such devices, the TEs use an alumina substrate (a good thermal conductor) for the hot and cold side end plates and copper or aluminum fins or blocks as heat exchangers on at least one side.
Thus, to a good approximation, conditions can be represented by the diagram in FIG. 5. In this case xcex94T has been split into the cold side at xcex94Tc and hot side xcex94Th where xcex94T=xcex94Tc+xcex94Th.
Using (1) and (2) in (5):                     β        =                                            q              c                                      q                              i                ⁢                                  xe2x80x83                                ⁢                n                                              =                                                    α                ⁢                                  xe2x80x83                                ⁢                I                ⁢                                  xe2x80x83                                ⁢                                  T                  c                                            -                                                1                  2                                ⁢                                  I                  2                                ⁢                R                            -                              K                ⁢                                  xe2x80x83                                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                T                                                                    α                ⁢                                  xe2x80x83                                ⁢                I                ⁢                                  xe2x80x83                                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                T                            +                                                I                  2                                ⁢                R                                                                        (        14        )            
But xcex94T is the sum of xcex94Tc and xcex94Th. So, for example, if xcex94Tc=xcex94Th then xcex94T=2xcex94Tc. Since the efficiency decreases with increasing xcex94T, it is highly desirable to make xcex94T as small as possible. One option is to have the fluid flowing by the hot side be very large compared to that by the cold side. For this case, the equation for heat flow from the hot side is:
qh=CpMxcex94Thxe2x80x83xe2x80x83(15) 
where CpM is the heat capacity of the fluid passing the hot side per unit time (e.g., per second).
Thus, if CpM is very large for a given required qh, xcex94Th will be very small. However, this has the disadvantage of requiring large fans or pumps and a large volume of waste fluid (that is, fluid not cooled, but exhausted as part of the process to achieve more efficient cooling).
A second option is to make the heat sink on the hot side very large so that the heat is dissipated passively. Examples would be a low power TE in a car with the hot side in very good thermal contact with the vehicle chassis, or a TE system in a submarine with the TE in good thermal contact with the hull and hence, the ocean water. In general, however, these methods are difficult to implement or cost, weight, size or other conditions limit their use. The result is that xcex94T is substantially larger than xcex94Tc in most devices, and efficiency suffers accordingly.
In general, an improved efficiency thermoelectric device is achieved by subdividing the overall assembly of thermoelectric elements into thermally isolated sub-assemblies. Overall efficiency may be improved by utilizing the thermal isolation, and controlling the positioning and direction of the flow of the material to be cooled or heated through portions of the thermoelectric device. Efficiency may also be improved, by varying xcex94T, and physical, thermal and electrical properties of portions of the overall thermoelectric device.
One aspect of the present invention involves a thermoelectric system for use with at least one medium to be cooled or heated. The system has a plurality of thermoelectric elements forming a thermoelectric array with a cooling side and a heating side; wherein the plurality of thermoelectric elements are substantially thermally isolated from each other in at least one direction across the array. At least one heat exchanger is provided on at least one of the cooling and/or the heating sides and in thermal communication with at least one thermoelectric element. The heat exchanger is configured to significantly maintain the thermal isolation of the thermoelectric elements.
In one embodiment, the medium, such as fluid, solids or a combination of both, moves across at least a portion of at least one side of the array, in at least one direction. In another embodiment, at least one characteristic, such as resistance, of the thermoelectric elements is varied in the direction of medium movement. Resistance may be varied in a number of ways, such as variation of length of the thermoelectric elements, variation of cross-sectional area of the thermoelectric elements, variations in the mechanical configuration of each thermoelectric element, or through resistivity of at least one thermoelectric material, and in any manner appropriate to the application.
In yet another embodiment, the current through the thermoelectric elements is different for at least some thermoelectric elements in the array.
Advantageously, the heat exchanger comprises a plurality of portions, such as posts, fins, or heat pipes, each portion in thermal communication with at least one thermoelectric element, at least some of the portions substantially thermally isolated from other of said portions in the direction of medium movement. Preferably, the thermal isolation of the portions corresponds to the thermal isolation of the thermoelectric elements, thereby providing significantly thermally isolated sub-assemblies. In one embodiment, a heat exchanger is provided on each of the cooling and the heating sides. Alternatively, one side has a heat sink and one side has a heat exchanger. The heat sink may be coupled to one side of the thermoelectric array via a heat pipe that is in thermal contact with the array at one end and with a heat sink at the other end. In another embodiment, the thermoelectric elements are also subjected to at least one magnetic field.
Advantageously, at least one characteristic of the thermoelectric system is dynamically adjustable through adjustment of the mechanical configuration of the thermoelectric system. A control system coupled to the thermoelectric system may adjust the mechanical configuration based upon at least one input to the control system. Preferably, the control system operates to improve efficiency dynamically through the adjustment. An algorithm may be provided in accordance with which the control system operates. In one embodiment, the control system adjusts at least one characteristic based upon at least one input to the control system.
The various features, such as thermal isolation, variation of a characteristic, variation of current, provision of magnetic fields and control systems may be used in various combinations, or alone, for particular applications.
Another aspect of the present invention involves a method of making a thermoelectric system for use with at least one medium, such as a fluid, solid or combination of fluid and solid, to be cooled or heated. The method involves the steps of forming a plurality of thermoelectric elements into a thermoelectric array with a cooling side and a heating side; wherein the plurality of thermoelectric elements are substantially thermally isolated from each other in at least one direction across the array, and exchanging heat from at least one side of the thermoelectric array in a manner that significantly maintains the thermal isolation of the thermoelectric elements.
In one embodiment of the method, the medium is moved across at least a portion of at least one side of the array in at least one direction. Another embodiment of the method involves the further step of varying at least one characteristic, such as resistance or mechanical configuration of the thermoelectric elements in the direction of medium movement. For example, resistance could be varied in any number of ways such as varying the length, the cross-sectional area, the mechanical configuration, or the resistivity of at least some of the thermoelectric elements. In one embodiment, the step of varying comprises dynamically adjusting the at least one characteristic. Preferably, the adjustment is in response to evaluation of at least one parameter from a sensory input or by a user. An algorithm may be followed to control the adjustment.
In one embodiment, the step of exchanging heat involves providing a heat exchanger comprising a plurality of portions, each portion in thermal communication with at least one thermoelectric element, at least some of the portions substantially thermally isolated from other portions in the direction of medium movement. The portions may take on any number of configurations such as posts, fins, or heat pipes, or other suitable heat exchanger materials. In one embodiment, the step of exchanging heat involves exchanging heat on both the cooling side and the heating side. Alternatively, the method involves the step of sinking heat from at least one side of the thermoelectric array.
In another embodiment, the method further involves the step of varying the current through at least some thermoelectric elements in the array. In yet another embodiment, the method further involves the step of subjecting the thermoelectric elements to at least one magnetic field.
These and other features of the present invention are described in further detail below in connection with a number of Figures.